The present application is a National Phase application of PCT/GB00/04270 filed on Nov. 7, 2000 and published in English, which claims priority from Application GB 9926519.1 filed on Nov. 10, 1999.
This invention relates to an insulation-piercing fastener, ie to a fastener intended for clamping engagement with an insulated electrical conductor in such a manner that the insulation is broken and electrical connection is established between the conductor and the fastener.
One known form of insulation-piercing fastener is intended for use with conductors having a stranded core. Such conductors commonly have insulation consisting of mineral oil-impregnated paper. The shank of this fastener has a generally cylindrical form, the tip of the fastener being formed with a diametric slot. When the tip of the fastener engages the insulated conductor the edges of the slot act as blades to cut away the insulation. Such fasteners generally cannot be used with conductors having insulation in the form of a plastics coating.
Another form of known fastener is suitable for use with plastics-insulated conductors, which generally have solid cores. This fastener has a tip formed as a circular blade. Such fasteners are not suitable for use with conductors having stranded cores and paper coatings since the area of contact between the conductor and the tip of the fastener is small and the blade tends to cut through the individual conductor strands.
The different forms of insulation-piercing fastener which must be used with different types of conductor means that a fitter must carry a supply of both. Problems can occur if the wrong fastener is used with any particular conductor.
There has now been devised an improved form of insulation-piercing fastener which overcomes or substantially mitigates the above mentioned or other disadvantages of the prior art.
According to the invention, there is provided an insulation-piercing fastener having a generally cylindrical shank, externally threaded along at least part of its length, the end portion of the shank which, in use, engages an insulated electrical conductor being formed with a hollowed end portion and the wall of said hollowed end portion having at least one interruption, the width of said at least one interruption being less than the maximum internal width of said hollowed end portion.
The fastener according to the invention is advantageous primarily in that it may be used satisfactorily with a range of different forms of insulated conductor, including conductors having solid cores and plastics insulation and conductors having stranded cores with mineral oil-impregnated paper insulation. The hollowed end portion of the fastener is able to accommodate a substantial quantity of insulation cut away from the conductor, whilst the contact area between the tip of the fastener and the conductor is sufficiently great to give good electrical contact.
The end portion of the shank will preferably be hollowed by virtue of having a recess formed in it, which recess will obviously be of somewhat lesser diameter than the shank itself. The recess may be a blind recess or may extend throughout the length of the fastener to form a bore. Whilst a bore of this form (ie a bore extending throughout the length of the fastener) may not be necessary for operation of the fastener it may be simpler to manufacture than a blind recess.
Also, at least part of the bore may be of non-circular cross-section for engagement with a drive component by which the fastener can be rotated. In such a case, the part of the bore that constitutes the hollowed end portion of the shank may be of circular cross-section or of non-circular cross-section, eg it may be a continuation of a bore of non-circular, eg square or hexagonal, cross-section formed for engagement with the drive component.
The size of the hollowed end portion relative to the overall cross-sectional area of the shank is preferably such that in the plane of the tip of the shank the hollowed end portion accounts for between 10% and 60% of the cross-sectional area of the shank.
The tip of the fastener is preferably formed with a pair of diametrically opposed interruptions which together define a slot extending across the end portion of the bolt.
The shank preferably has a diameter in the range 3 to 30 mm. The dimensions of the interruption(s) in the wall of the hollowed end portion, and the number of such interruptions, will generally be commensurate with the size of the fastener, and with the nature of the insulated conductor with which it is to be used. Typically, the interruption is between 0.5 and 5 mm in width and has a depth of between 0.5 and 20 mm.
The fastener according to the invention may be manufactured in any material which is electrically conductive and has the requisite mechanical properties. Most preferably, the bolt is formed in brass or copper.
According to another aspect of the invention, there is provided an electrical connector including a socket adapted to receive an electrical conductor, a wall of said socket having a threaded bore in which an insulation-piercing fastener as defined above is received.